Hundreds of studies have explored the benefits and possible fallbacks of high-intensity interval training (HIIT): a form of training that uses repeated high-intensity exercise bouts interspersed with brief recovery periods.
Work periods during HIIT, at least in studies, generally include bodyweight exercises, running, and cycling. Work to rest ratios vary depending on fitness level, but beginners should start with a 1:1: work to rest ratio and performance-oriented athletes should try a 2:1 or 3:1 work to rest ratio.
This article will review the latest research on the topic (nothing older than 2013) and provide a general conclusion about whether or not doing HIIT is superior to regular steady state cardio.
If you take a HIIT class, the instructor is likely to explain that the class will result in excess post oxygen consumption (EPOC) or oxygen debt, which is the oxygen you breathe in above resting values used to restore the body to the pre-exercise condition. They are correct in stating that you’ll burn more calories after your workout is complete—up to 24 hours post-workout—but this likely won’t have an effect on fat loss unless you’re doing HIIT for months.
In fact, the study we’ll explore first found that both HIIT and steady state cardio burn fat to the same extent.
LESS INTENSE = MORE FAT LOSS?
A 2017 Obesity Reviews meta-analysis of 31 studies explored the effects of HIIT, sprint interval training, and moderate intensity continuous training on body adiposity (body fat). The researchers found that while all three types of training decreased body fat percentage on average, there were no significant differences between HIIT, sprint training or steady state cardio for any body fat outcome. Even more interesting is that the researchers concluded that none of these exercise forms resulted in clinically meaningful reductions of body fat, as study participants only lost about 1% body fat in the studies.
How could a meta-analysis of 31 studies with a minimum 4-week exercise intervention result in a finding that HIIT doesn’t really burn that much fat? Well, clinical trials involving exercise don’t always provide dietary restrictions, and if they do, the researchers often do a poor job of actually monitoring what people eat. For example, study subjects are often asked to fill out food logs during diet studies which can be inaccurate because people forget what they eat or simply don’t disclose everything they eat. Therefore, it could be the case that HIIT without dieting doesn’t do much for body fat. Plus, the study subjects probably varied greatly in their genetics and metabolic profile and everyone loses body fat in different ways.
So what has HIIT been shown to do in the literature? For one, it keeps you young.
HIIT FOR LONGEVITY
In a 2017 Cell Metabolism study, 12 weeks of HIIT performed by young (about 25 years old) and older (about 70 years old) adults resulted in increased VO2max, insulin sensitivity, mitochondrial function, FFM, and muscle strength. VO2max is the highest amount of oxygen an athlete can use during exercise. Insulin sensitivity is the pancreas’ ability to release insulin, a nutrient-absorbing hormone, into the bloodstream to move sugar to our organs/muscles. Insulin sensitivity is a good thing for preventing weight gain and diabetes. Mitochondria are the powerhouses of cells that play a role in producing ATP, the body’s primary source of energy. HIIT improved the oxidative capacity, or efficiency, of mitochondria regardless of the person’s age. In comparison, the strength training only and combined strength/steady state cardio groups in this study didn’t see such robust cellular benefits in both age groups.
Not only does HIIT increase longevity, it’s more enjoyable for some people too.
HIIT MAKES YOU HAPPIER THAN STEADY STATE CARDIO
According to a small 2017 PLOS ONE study, three sessions of HIIT was more enjoyable than three sessions of moderate intensity continuous cardio in healthy young adults, even though the HIIT had higher levels of perceived exertion, exercise heart rate and blood lactate. In this study, HIIT consisted of eight bouts of 60 seconds of cycling at 85% of maximum effort separated by 60 seconds of recovery at 25% maximum effort. The moderate intensity protocol called for 20 minutes of cycling at 45% of maximum effort.
After doing the exercises tests, the subjects took the 18-question Physical Activity Enjoyment Scale (PACES) test to assess just how much they liked each training style. It turned out that 92% of participants demonstrated high enjoyment and preference for HIIT compared to steady state workouts.
A 2016 PLOS ONE study examined the topic of HIIT making you happier in even further detail. In that study, young adults did six weeks of either HIIT or moderate intensity training and while both groups reported similar feelings at week one, by week 4, there was a tip in favor of enjoying HIIT more. By week five, HIIT was significantly more enjoyable than moderate intensity training. The average HIIT cycling session used a 1:1: work to rest ratio at 90-95% maximum heart rate (MHR) during work while the moderate intensity was a 27.5-minute cycling session at 70-75% MHR. What was really interesting in this research was that the HIIT workouts got harder over time during the six weeks and as they did, the participants started to report more enjoyment. This suggests that conquering new physical challenges while training for them is something people like. Go figure.
HIIT FOR ATHLETIC PERFORMANCE
This is where the research gets really dense and scientific. There are studies about athletes from various sports that do HIIT but the most frequent and popular implementation is in the endurance world. Runners/cyclists/triathletes have been experimenting with HIIT to shave their race times and while each athlete may get a different response, a 2013 Sports Medicine review outlined the effect of HIIT on endurance performance in great detail. The excerpt below explains just how intense exercise should be for athletic performance, specifically that there is a bell-shaped curved in relation to exercise intensity and muscular performance.
“There is likely a bell-shaped relationship between the intensity of a HIT session and the acute neuromuscular performance, with too low and too high (all-out) intensities having not enough and acute detrimental effects, respectively. Work intensities at 80–85 % VO2max require recruitment of fast twitch fibers, induce post-activation potentiation and possibly lead to long-term structural adaptations that allow fatigue-resistance to high-speed running. In contrast, supramaximal-to-maximal (120 % >VO2max ) intensity exercises are likely associated with acute impairments in muscular performance.”
HIIT AND YOUNG OVERWEIGHT FEMALES
One novel 2016 Applied Physiology, Nutrition & Metabolism study sought to compare the effects of a HIIT regiment alone and a HIIT regiment that included a plyometrics routine performed prior to the HIIT. In this study, 68 obese female adolescents did either 12 weeks of HIIT running on an outdoor track or that same protocol preceded by plyometric exercises such as double-leg jump, medicine ball overhead throw, hurdle hops, and medicine ball partner push pass. The HIIT called for a 1:1 work to rest ratio of 30 seconds performed at a 100% of VO2max during work and 50% of VO2max at recovery.
After the 12 weeks of training, the ladies that did the plyometrics+HIIT showed significantly less plasma glucose levels, better squat jump performance, improved insulin sensitivity and greater improvements in lean body mass compared to the HIIT-only women. Leptin, a hormone that decreases the rate of glucose uptake, was decreased more so in the plyo+HIIT group which is likely the reason for less sugar in the blood after the study.
When we look at the effect of HIIT vs moderate-intensity aerobic exercise within the young, obese female population, unsurprisingly, HIIT is more enjoyable. According to a 2016 PLOS ONE study, five weeks of either HIIT or moderate intensity cardio resulted in similar losses in body fat and total body weight and similar improvements in VO2max. The HIIT testing protocol in this study was 8 seconds of hard cycling interspersed with 12 seconds of passive rest for 20 minutes while the moderate cyclers worked at 60-80% of VO2max for 40 minutes. The women took that same PACES enjoyment rest as in the study mentioned earlier and of course, they liked HIIT more than steady state cardio. The main findings here was that HIIT and steady state cardio elicit similar metabolic changes while HIIT does it in less time and was more enjoyable.
HIIT AND YOUNG OVERWEIGHT MALES
Researchers at the University of Alabama found a different result when they studied the effect of five weeks of HIIT and moderate intensity cycling on young, overweight men. In their 2016 PLOS ONE study, the steady-state cardio resulted in significantly better VO2max compared to HIIT. The reason for this might have been the unique timing of the HIIT test which saw the guys ride for four minutes at 15% of maximum anaerobic power, then 30 seconds at 85% maximum power, repeated for four rounds. At the end of the four rounds, they cycled for two minutes at 15% of maximum power again. The moderate intensity group cycled for 45-60 minutes at 55-65% of VO2max. Both training protocols resulted in improved insulin sensitivity, reduced blood lipids, and decreased percent body fat but the moderate intensity group saw better cardiovascular fitness.
This study suggests that one might need an aerobic base already to see the superior fitness improvements from HIIT as compared to steady state aerobic training. The results also might suggest that true HIIT doesn’t include four minutes of submaximal work in any fashion.
There’s a lot of research on HIIT and this article reviews only the last four years worth of peer-review studies in scientific journals. In the end, the effect of HIIT compared to steady state cardio seems to vary based on gender within the overweight population, with overweight males not seeing as much of a cardiovascular effect as females do. HIIT does help endurance athletes perform faster, but programming those workouts is a complex puzzle that starts with knowing that 80-85% of VO2max is the sweet spot for muscular performance. In the general population, both HIIT and steady-state cardio training programs DO burn fat, even if it’s scientifically insignificant in terms of overall health.
The lesson here is that fat loss should not be the only thing we look for in a workout. Dozens of study participants found HIIT more fun than steady state cardio despite the fact that it’s harder. Apparently, some of the same people that are experimenting with extreme fitness and super high-intensity classes are also participating in studies about HIIT. Or, random samples of society actually have mental grit and find pleasure in overcoming tough tasks. When it comes to HIIT, the concept is simple: choose five exercises or a cardio mode, work for 60 seconds, rest for 60 seconds, repeat four times, resting 90 seconds between rounds. The effects are more complicated but we do know that you’ll be sweating when you’re done.